Television system



Dec. 1, 1942:

E. L c. WHITE TELEVISION SYSTEM Filed May 10, 1939 SYNcM 516N440INVENTOR ERIC ZAWREIVCE CASH/V6 WHITE ATTORNEY which constitutes one ofthe whiter than Patented Dec. 1, 1942 iilTED STATES PATENT ICETELEVISION SYSTEM Eric Lawrence Casling White, Hillingdon, Middlesex,England, assignor to Electric & Musical Industries Limited,

Hayes, Middlesex, England,

a company of Great Britain Application May 10, 1939, Serial No.

In Great Britain May 18, 1938 '4 Claims.

917, filed May 11, 1939, in which a method of obi taining a rapidlyacting automatic gain control as required for example in aircrafttelevision sys tems is described, although the method is applicable totelevision systems generally. In carrying out the method of UnitedStates application Ser. No. 141,917, filed May 11, 1937, the waveformtransmitted is a carrier wave modulated by picture signals and includessets of both whiter than white and blacker than black signal pulseswhich occur at predetermined intervals, and the receiver is providedwith an automatic gain control system which is designed to be responsiveto the difierence between the amplitude of said signal pulses, thereceiver being also provided with a synchronizing system which isdesigned to heresponsive to one set of the signal pulses. The blackerthan black pulses constitute line-synchronizing signals and each of suchpulses forms a portion of a double pulse, the other portion of whitepulses. The carrierwave is also modulated by frame synchronizing signalswhich modulate the carrier wave in the whiter than white sense, the

frame signals being interrupted by the line synchronizing signals in'theblacker than black sense so that automatic gain control potentials andline synchronizing signals can be obtained during the occurrence of thefram synchronizing signals.

According to the present invention, a modlfication of the invention setforth in the specification of United States application Ser. No. 141,917filed May 11, 1937, consists in modulating the tted carrier wave withframe synchroniz- 111g Si nals in the blacker than black sense. In orderto eliminate at the receiver undesirable effects due to the pulses whichmodulate the carrier wave in the whiter than white sense, such pulsesare suppressed by mixing them with the line synchronizing signals whichare separated, delayed and/or broadened for this purpose.

In a particular television transmitting and receiving system embodyingthe invention the waveform sent out by the transmitter is a carrier wavemodulated by picture signalsin such a sense that an increase in picturebrightness is represented by an increas in carrier amplitude, saidwaveform including blacker than black line and frame auxiliary signalsand wherein the receiver is provided with an automatic gain controlsystem which is designed to be responsive to the absolute amplitude ofsaid whiter than white auxiliary signals.

In order that the invention may be more clearly understood and readilycarried into effect a waveform modulated in accordance with theinvention will now be more" fully described by way'of example withreference to the drawing filed with the specification in which Fig. 1shows such a waveform, and

Fig. 2 shows a circuit arrangement for preparing the line synchronizingsignals to efiect suppression of the gain controlling pulses.

If the waveform described in the specification of application Ser. No.141,917 filed May 11, 1937, is changed so that the frame synchronisingpulses modulate the carrier wave in the blacker than black sense withoutother modification, a waveform results having th disadvantage that theline synchronizing pulses are interrupted by the synchronizing signalsand whiter than white 55 cause disturbance in the line framesynchronizing pulses and this is apt to scanning at a receiver. Animprovement is effected if the same step is adopted and in addition theautomatic gain control pulses which modulate the carrier wave in thewhiter than White sense are made to precede instead of to follow theline synchronizing pulses which modulate the carrier wave in the,blacker than black sense. In this case the trailing edge of a gaincontrol pulse will coincide with the leading edge of the linesynchronizing pulse and since the automatic gain controlling pulsescontinue throughout the frame synchronizing pulses, the line scanningoscillator at the receiver will continue to be synchronized by thetrailing edges of the automatic gain controlling pulses.

Since the automatic gain control pulses occur I before the linesynchronizing pulses, they result in a bright vertical bar appearing onthe extreme right-hand edge of the received picture and this can beeasily masked off. However, this does represent a loss of time duringwhich picture signals could otherwise be transmitted and the waveformshown in Fig. 1 of the accompanying drawing overcomes this disadvantage.

Referring to Fig. l, the waveform shown is for a line frequency of10,000 per second, and it will be necessary that the automatic gaincontrol pulses I, whichare of 10 micro-seconds duration, follow the linesynchronizing pulses 2 as in the waveform described in the specificationof application Ser. No. 141,917 filed May 11, 1937, the

duration of these line pulses being 5 micro-seconds. e framesynchronizing pulses 3 consist of 10 to broadened line synchronizingpulses but with the automatic gain control pulses continuing throughthem. The picture signals 4, the duration of each line of which is 85micro-seconds, are prevented from coming below a level represented bythe line 5 which is of the order of 30% of the peak carrier wave output,and it is also convenient but not necessary to limit the upwardexcursions of the picture signals to about 90% as represented by thedotted line 6. 00% modul tion of the carrier wave is represented by theline I. The picture signals are suppressed to the 30% level 5 during theperiod of the frame synchronizing pulses.

The effect oi the automatic gain control pulses of Fig. 1 on thereceived picture, which would be to brighten the end of the returnstroke and the beginning of the working stroke, can be suppressed byseparating the synchronizing pulses, delaying them by an amount notquite equal to their width, broadening them to a breadth slightlygreater than the automatic gain control pulses and mixing them thepicture signals.

A circuit arrangement for effecting the delaying and broadening ofseparated line synchronizing pulses is shown in Fig. 2. The separatednegative line synchronizing pulses are applied to the terminals 8between which and a diode valve D1 is a network giving an initial delay151. The

delays introduced by the following networks are each equal to t2 andlessthan the width of the original pulse. These delay networks areconnected to diode valves D2, D3 and D4, the anodes of which areconnected together and with that of D1. The output from the diodes ispassed to a valve V, the output from which consists of delayed andbroadened positive pulses.

In operation, the grid of the valve V is maintained negative so long asincoming pulses are applied through the delay network to any ofthediodes D1 to D4. It should be arranged that the amplitude of theincoming line synchronizing pulses is 'sufiicient to cause the valve Vto be shut off so that the waveform of the output is regular.

Although four diodes are shown, any number n greater than one may beused. The width of the output pulse will be increased by (12-1) is.Thus, if the width of the line synchronizing pulse is t5 and that of theautomatic gain control pulse is t it must be arranged that t is greaterthan n and t1+(n-l)ta is greater than t.+t The delayed and broadenedpulse obtained from in negative sense with asoaoes the valve V willoverlap the automatic gain control pulse on both sides and willtherefore suppress it if mixed in the negative sense;

The waveform described possesses the advantages that all the positiveautomatic gain control pulses are equal in length so that design of arapidly acting automatic gain control circuit at the receiver isfacilitated and it is not necessary to arrange for blacking out theframe pulse at "the receiver.

While in the waveform described the line synchronizing pulses have beenshown to be shorter than the automatic gain control pu1ses,,they may ifdesired be of the same length or longer than the automatic gain controlpulses. In either of these latter cases, the suppressing pulses may beobtained merely by applying the line synchronizing pulses to a delaynetwork.

What I claim is:

1. In a television system wherein line and frame synchronizing impulsesof the same polarity are transmitted between sets of video signals, andwherein a signal of opposing polarity is transmitted after each linesynchronizing signal, the method which comprises the steps of separatingthe synchronizing pulses, delaying the pulses by an amount differingfrom their duration, broadening the pulses to a breadth slightly greaterthan the width of the pulses having an opposing polarity thereto, andmixing the pulses in a negative sense with the video signals.

2. In a television transmitting system wherein automatic volume controlsignals are transmitted at a time adjacent the transmission of linesynchronizing signals which are of opposite polarity thereto, and inwhich frame synchronizing signals network means serially connected, eachof said delay network means having a diode connected substantiallyacross the terminals thereof.

ERIC LAWRENCE CASLING WHITE.

